1. Field of the Invention
The present invention relates generally to integral polymer geogrids and other oriented grids used for structural or construction reinforcement purposes. More particularly, the present invention relates to such integral polymer grids made from homopolymer and copolymer polyethylene terephthalate (“PET”) in order to achieve a higher tensile strength to weight ratio and a higher creep reduced strength to weight ratio.
This invention also relates to the method of producing such integral PET grids. Lastly, the present invention relates to the use of such integral PET grids for soil reinforcement and methods of such reinforcement.
For the purpose of this invention, the terms “integral PET grid” and “integral PET grids” are intended to include integral polyethylene terephthalate geogrids and other integral polyethylene terephthalate grid structures made by orienting (stretching) starting materials in the form of sheets or the like having holes or depressions made or formed therein.
2. Description of the Prior Art
Polymeric integral grid structures having mesh openings defined by various geometric patterns of substantially parallel, orientated strands and junctions therebetween, such as geogrids, have been manufactured for over 25 years. Such grids are manufactured by extruding an integrally cast sheet which is subjected to a defined pattern of holes or depressions followed by the controlled uniaxial and biaxial orientation of the sheet into strands and junctions defined by mesh openings formed by the holes or depressions. Orienting the sheet in either the uniaxial or biaxial direction develops strand tensile strength and modulus in the corresponding direction. These integral oriented polymer grid structures can be used for retaining or stabilizing particulate material of any suitable form, such as soil, earth, sand, clay, gravel, etc. and in any suitable location, such as on the side of a road or other cutting or embankment, beneath a road surface, runway surface, etc.
Various shapes and patterns of holes have been experimented with to achieve higher levels of strength to weight ratio, or to achieve faster processing speeds during the manufacturing process. Orientation is accomplished under controlled temperatures and strain rates. Some of the variables in this process include draw ratio, molecular weight, molecular weight distribution, and degree of branching or cross linking of the polymer. As a result of the orientation process, the finished product has a much higher tensile modulus and a highly reduced creep sensitivity.
The manufacture and use of such geogrid and other integral polymer grid structures can be accomplished by well-known techniques. As described in detail in U.S. Pat. No. 4,374,798 to Mercer et al. (“Production of Plastic Mesh Structure”), U.S. Pat. No. 5,419,659 to Mercer et al. (“Plastic Material Mesh Structure”), U.S. Pat. No. 4,590,029 to Mercer et al. (“Molecularly Orienting Plastics Material”), U.S. Pat. No. 4,743,486 to Mercer and Martin (“Product and Method of Producing a Plastics Material Mesh Structure”), and U.S. Pat. No. 4,756,946 to Mercer (“Plastic Material Mesh Structure”), a starting polymeric sheet material is first extruded and then punched to form the requisite defined pattern of holes or depressions.
As disclosed in the aforesaid patents, the starting sheet material is uniplanar or substantially uniplanar. As further described in the aforesaid patents, the uniplanar or substantially uniplanar punched starting material can be stretched only in the machine direction whereby the polymeric material between the punched holes is stretched to form highly molecularly oriented parallel strands interconnected by parallel transverse bars substantially at right angles to the strands. The stretching is continued so that the molecular orientation extends into the mostly unoriented transverse bar, which forms the junctions between the aligned strands. This uniaxial stretching of the punched starting material forms a uniaxial integral mesh structure, or uniaxial integral geogrid. The uniaxial integral geogrid is substantially uniplanar and has a plurality of highly oriented parallel strands that are interconnected by partially oriented junctions in the transverse bar, all substantially symmetrical about a median plane. The highly oriented parallel strands and the parallel transverse bars form an array of longitudinal openings between the parallel strands.
As further described in the aforesaid patents, when the substantially uniplanar starting material is biaxially stretched, i.e., first in the machine direction and then in the transverse direction, the stretching forms a biaxial integral mesh structure, or biaxial integral geogrid. The biaxial integral geogrid is also substantially uniplanar and has a plurality of highly oriented strands interconnected by partially oriented junctions, all substantially symmetrical about a median plane. The highly oriented strands and the partially oriented junctions define an array of mesh openings in the biaxial integral geogrid.
When imparting the high molecular orientation to the strands of the biaxial integral geogrid during the biaxial stretching, the molecular orientation is caused to extend into the junctions and around the crotch of the partially oriented junctions between adjacent oriented strands.
It is intended that the present invention be applicable to all integral PET grids regardless of the method of forming the starting material or orienting the starting material into the geogrid or grid structure. The subject matter of the foregoing patents is expressly incorporated into this specification by reference as if the patents were set forth herein in their entireties. These patents are cited as illustrative, and are not considered to be inclusive, or to exclude other techniques known in the art for the production of integral polymer grid materials.
Traditionally, the polymeric materials used in the production of integral grids have been high molecular weight homopolymer or copolymer polypropylene, and high density, high molecular weight polyethylene. Various additives, such as ultraviolet light inhibitors, carbon black, processing aids, etc., are added to these polymers to achieve desired effects in the finished product.
While the conventional polypropylene and polyethylene materials exhibit generally satisfactory properties, it is structurally and economically advantageous to produce a grid material having a higher tensile strength to weight ratio and a higher creep reduced strength to weight ratio.
Creep is the process by which the dimensions of a material change with time, while subjected to a sustained or variable load. See, e.g., FIGS. 4 and 5. FIG. 4 illustrates comparative creep curves for various conventional polymeric geotextile materials such as polypropylene and polyester under loading of 40% ultimate strength. FIG. 5 shows representative creep curves for filaments of various polymers.
The creep behavior of a synthetic material is a function not only of polymer type and physical structure, but also of factors such as geometric structure (e.g., woven, nonwoven, integral grid, etc.), surrounding medium, environmental temperature, presence of any micro or macro damage, and aging.
With regard to the temperature factor, polymers creep significantly when exposed to stress above their glass transition temperature, Tg. This means that high density polyethylene (“HDPE”) and polypropylene (“PP”), with a Tg of −120° C. and −18° C., respectively, creep substantially more at ambient temperature then does PET, which has a Tg of 69° C.
Another method of manufacturing grids employs weaving or knitting technology. High tenacity filaments of either PET or polypropylene are twisted together to form yarn. These yarns are either woven or knitted into open structured fabric and then coated with a protective coating to provide protection to the core yarns. The protective coating can be, for example, polyvinyl chloride (PVC), a bituminous substance, or latex. The primary difference between products manufactured using this technology and the aforementioned integral grids is that the woven or knitted products have flexible junctions with considerably low junction strength. Some examples of such grids are the geogrids manufactured by companies such as Merex, Huesker, and Strata. FIG. 13 of the drawings summarizes basic properties of various polymers typically used to produce staple fibers, continuous filaments, and oriented tapes.
Still another method of manufacturing grids uses highly oriented ribs or straps of polyester. The ribs are processed in a welding device in which cross machine direction ribs are introduced and welded together forming dimensional apertures. An example of such a grid is the Secugrid® manufactured by NAUE Gmbh & Co. The junction strength of products manufactured using this method tends to fall between that of integral grids and woven/knit grids.
Therefore, a need exists for an integral polymer material that not only is suitable for use in geogrid service, but that exhibits a higher tensile strength to weight ratio and a higher creep reduced strength to weight ratio than those values associated with conventional geogrid materials.